1. Field Of The Invention
This invention relates to release liners, and more particularly, to release liners suitable for use with highly aggressive pressure-sensitive adhesives.
2. Discussion of the Art
Pressure-sensitive adhesive tape, which is typically wound into rolls, has a low-adhesion backsize coating applied to the major surface of the backing not bearing adhesive to permit the tape to be unwound without delamination and transfer of adhesive to the surface of the backing not bearing adhesive. The force required to separate the adhesive layer from the low-adhesion backsize coating, i.e., peel force, ranges from 6 to 35 Newtons per decimeter (N/dm) per unit width. If the tape is not wound into a roll, its adhesive layer is customarily protected by a disposable web that bears a low-adhesion coating to which normally tacky and pressure-sensitive adhesives adhere very weakly, e.g., a peel force ranging from about 0.2 to about 6 N/dm, preferably 0.2 to 2 N/dm. Release coatings having peel forces in this preferred range of 0.2 to 2 N/dm are hereinafter referred to as "premium release" coatings.
Release coatings must both adhere strongly to the backing and be sufficiently cured or rendered incompatible with the adhesive so that it does not transfer to the adhesive layer and adversely affect adhesive power of the adhesive. Low adhesion backsize coatings having such properties are disclosed in U.S. Pat. Nos. 3,318,852; 3,536,749; 4,057,596; and 4,216,252.
Certain pressure-sensitive adhesives, particularly those derived from silicone resins, are so aggressive that tapes employing them exhibit undesirably high peel forces upon removal from known low-adhesion coatings, especially after prolonged storage or when used in a high-temperature environment. The adhesive of such tapes may carry away, and consequently, be contaminated by appreciable amounts of the low-adhesion material. If this occurs, the tapes will exhibit poor readhesion when applied to "end-use" receptor surfaces.
Known low-adhesion coatings, e.g. poly(urethanes), poly(tetrafluoroethylenes), and poly(dimethylsiloxanes), do not provide release peel forces in the range 0.2 to 2 N/dm when used with poly(dimethylsiloxane) adhesives.
The utility of polymeric fluorocarbons as release coatings has been disclosed in U.S. Pat. Nos. 4,472,480; 4,567,073; and 4,614,667. Uncrosslinked fluorinated waxes and oils are frequently used as lubricants and mold release agents in applications where low adhesion coatings are designed to fail cohesively within the bulk phase. Conversely, crosslinked fluorinated waxes and oils are preferred when it is desired to avoid cohesive failure of release liners. Examples of crosslinking reactions that have been used to prepare low energy release liners are photochemical crosslinking, described in U.S. Pat. Nos. 4,472,480; 4,567,073; and 4,614,667; thermal crosslinking of silicone resins, described in U.S. Pat. No. 4,472,480; and chemical crosslinking of silicone resins (hydrosilation), described in U.S. Pat. No. 4,565,741. U.S. Pat. No. 4,321,404 describes abherent coatings made by using electron beam radiation to polymerize a mixture of film-forming and polyfluorinated oligomers in a single step. The mixture is formulated so that the fluorinated materials tend to segregate at the coating surface, i.e., the air interface, so that the abherent coating can be crosslinked with a single pass through the electron beam curtain. These coating,#have excellent readhesion; however, they were formulated for use with acrylic adhesives and typically have high release values when used with these adhesives. These release values would be higher still if these abherent coatings were used with the highly aggressive silicone pressure-sensitive adhesives.
In order to further reduce the possibility of cohesive failure, a crosslinked primer coating to securely anchor the release coating to its backing has been used. U.S. Pat. Nos. 4,567,073 and 4,614,667 disclose the advantage of first photopolymerizing a thin layer of a film-forming polymer that is strongly bonded to the surface of a substrate. This primer layer is also capable of subsequently reacting with functional groups attached to a polyfluoropolyether, which is applied as a second thin layer over the photopolymerized layer, thereby chemically bonding the release coating to the inner primer layer. These release liners exhibit release of about 0.6 N/dm, and excellent readhesion for certain classes of silicone-based pressure-sensitive adhesives.